Creating a Well-focused Laser-accelerated Proton Beam as a Driver for Proton Fast Ignition

Density maps of protons at t=7.3 ps for the case of a) free-standing partial hemisphere target, and b) a cone structure target. Simulations illustrate the bending of the trajectories near the axis and away from the inner surface of the cone. Dashed lines follow trajectories to t=19 ps. Red dashed lines include final energies and demonstrate that more energetic particles are emitted closer to the axis.

The Science

Developed a new understanding of how a quasi-neutral proton jet focuses and propagates though a focusing cone structure. Electric fields in both the proton jet and along the surface of the conical structure play a crucial role in controlling the focusing.

The Impact

An important requirement for the success of proton fast ignition is the generation of beams that are intensely focused, down to a diameter of tens of microns. This work demonstrates the ability to generate and focus a short-pulse laser accelerated proton beam to these approximate requirements, having the necessary 5-15 MeV energy spectrum, and in a conical geometry of relevance to proton Fast Ignition.

Summary

Recent progress in generating high energy (> 50 MeV) protons from intense laser-matter interactions (1018 – 1021 W cm-2) has opened new areas of research, with applications in medical imaging, astrophysics, high energy density physics, and ion-proton beam fast ignition Inertial Fusion Energy. Rapid advances in these areas will be driven by improved focusing technologies. This research reports the first investigation of the generation and focusing of a proton beam using a novel cone-shaped design, relevant to conceptual proton fast ignition. In a Nature Physics article and subsequent theory and simulation papers, it is demonstrated that focusing is strongly affected by the electric fields in the proton jet. It is also found that with a cone target, the sheath electric field effectively “channels” the proton beam through the open cone tip, substantially improving the focusing properties. These results agree well with both theory and particle simulations and are critical for providing the physics basis for developing future target designs for proton fast ignition inertial fusion energy.